Processes linked to shallow subduction, slab rollback, and extension are recorded in the whole-rock major-, trace-element, and Sr, Nd, and Pb isotopic compositions of mafic magmatic rocks in both time and space over southwestern United States. Eocene to Mio-Pliocene volcanic rocks were sampled along a transect across the west-central Great Basin (GB) in Nevada to the Ancestral Cascade Arc (ACA) in the northern Sierra Nevada, California (~39°–40° latitude), which are interpreted to represent a critical segment of a magmatic sweep that occurred as a result of subduction from east-northeast convergence between the Farallon and North American plates and extension related to the change from a convergent to a transform margin along the western edge of North America.
Mafic volcanic rocks from the study area can be spatially divided into three broad regions: GB (5–35 Ma), eastern ACA, and western ACA (2.5– 16 Ma). The volcanic products are dominantly calc-alkalic but transition to alkalic toward the east. Great Basin lavas erupted far inland from the continental margin and have higher K, P, Ti, and La/Sm as well as lower (Sr/P)pmn, Th/Rb, and Ba/Nb compared to ACA lavas. Higher Pb isotopic values, combined with lower Ce/Ce* and high Th/Nb ratios in some ACA lavas, are interpreted to come from slab sediment. Mafic lavas from the GB and ACA have overlapping 87Sr/86Sr and 143Nd/144Nd values that are consistent with mantle wedge melts mixing with a subduction-modified lithospheric mantle source. Eastern and western ACA lavas largely overlap in age and elemental and isotopic composition, with the exception of a small subset of lavas from the westernmost ACA region; these lavas show lower 87Sr/86Sr at a given 143Nd/144Nd. Results show that although extension contributes to melting in some regions (e.g., selected lavas in the GB and Pyramid Lake), chemical signatures for most mafic melts are dominated by subduction-related mantle wedge and a lithospheric mantle component.